1. Field of the Invention
This invention relates to an air filter, and more particularly to an air filter of a combination of electret filters and non-electret filters to have a high efficiency for removing particles, dust, etc. in the air.
The term "electret" is employed throughout the specification to denote an electrified substance which exhibits electrical charges of opposite sign on opposite faces, the electrification being throughout the entire volume of the substance, rather than merely on its surface.
2. Prior Art
Recently, there has been an increasing demand for air filters of high efficiency which can remove even undesirable particles such as ultrafine corpuscles out of the air. The demand is raised particularly in factories for manufacturing IC (integrated circuits) and LSI (large-scale integrated circuits), operation rooms and treatment rooms in hospitals, and preparation rooms for medicine and the like. In such places, the so-called clean room system has conventionally been employed to minimize entry of dust or microbes brought by people entering the rooms and so forth, so that the air to be supplied into the rooms is first filtered for purification by air filters and then blasted from coiling (vertical laminar flow system) or through walls (horizontal laminar flow system).
However, especially in the field of IC and LSI production, further development toward still smaller size, and higher density and precision has been under way, and for the improvements of quality and performance, accuracy in insulation at a gap less than 0.3 .mu.m is brought into question, while in hospitals also, attention is now directed to microbes less than 0.3 .mu.m in size. Thus, removal of these ultrafine particles has been strongly required. More specifically, the conventional clean rooms which are provdied with clean air projectors equipped with conventional high efficiency air filters (HEPA filter) having collecting efficiency higher than 99.97% for the fine particles on the order of 0.3 .mu.m in diameter, the cleanness does not meet requirements, because it shows only class 1 or 100 at most (i.e. the number of particles in 1 ft of air is 1 or 100 respectively, on the basis of U.S. Federal Standards 209b) with respect to the fine particles having diameter larger than 0.5 .mu.m. Therefore, the higher grade clean room, i.e. the so-called super clean room of class 1 or 100 to fine particles smaller than 0.3 .mu.m, for example to ultrafine particles or 0.1 .mu.m has come to be required.
An electrostatic precipitator or electrostatic filter conventionally available which might be employed for the requirement described above is not only high in the cost of installation of power sources, but insufficient in dust removing efficiency against the ultrafine particles.
Moreover, only one sheet of HEPA filter does not provide sufficient dust removal efficiency against the ultra-fine particles. In order to overcome the disadvantages as described above, it was naturally considered to improve the dust collecting efficiency either by piling up more than two sheets of the filter or by increasing the density thereof. In this case however, although the dust removing efficiency is improved, the pressure loss becomes very large. In addition, not only facilities such as blowers, ducts, etc. become bulky, but power charge must be increased, which is not practical or reliable.
In the above connection, since the conventional high efficiency air filter depends on particle collection by mechanical filtering effect, it may be possible to collect fine particles if a gap between the fibers is further reduced, or if the filter has smaller pores in diameter. However, even by the above arrangement, the drawbacks as described earlier can not be avoided, while filter clogging tends to take place, causing pressure loss, and resulting in a short life of the filter.